One Of the more important processes for applying coating materials to commercial products is the use of electrostatic resin coating techniques. Briefly, this technique involves spraying electrically charged powder paint particles to a workpiece which is at ground potential. The charged powder particles are attracted to the workpiece and form a uniform coating therearound which is subsequently fused by the application of heat to form a solid homogeneous coating.
In the electrostatic powder coating process, relatively large portions of the released electrostatically charged powder do not impinge upon the workpiece or do not have sufficient adherence thereto, because the desired layer thickness has already been attained on the workpiece which neutralizes the charge. The workpieces are generally sprayed within a spray booth which contains the toxic powder. The booth has openings that allow air to be drawn in, so that the excess powder does not drift out of the spray booth enclosure. This air is then exhausted out of the spray booth after having gone through a filter to remove the oversprayed powder from the air. It is desirable to filter out the excess powder and possibly reuse the resin to reapply onto new workpieces.
U.S. Pat. No. 3,918,641 issued to Lehmann discloses an installation in which the excess powder or overspray is continuously collected. The electrostatic powder coating installation depicted therein, consists of a spray booth that has a spray gun 17 that sprays electrostatically charged powder onto workpieces. The booth has an inner cavity that is bound by the first face of a filter belt 23 moving through the cavity. The filter belt moves between two roller drums located at either end of the spray booth. A vacuum 44 is located above the filter belt that removes the excess powder from the first face of the belt.
In this installation, the excess powder and exhaust air within the inner cavity are drawn to the filter belt under the influence of an air suction device. The filter belt has a predetermined permeability such that the air is purified of the excess powder as it is drawn through the filter belt, wherein the powder is collected on the surface of the filter belt and recovered by the vacuum for reapplication by the spray gun.
U.S. Pat. No. 4,257,345 issued to Brice discloses a similar installation as '641, but has the added feature of a second movable belt used to protect the filter belt from abrasive contact with a wire grate interposed between the belt and the rollers of the belt assembly.
U.S. Pat. No. 4,153,008 issued to Marino discloses a movable filter belt comprised of two layers, one which collects the excess powder and a second coarser layer to provide the first layer with support. Means are provided adjacent the powder cleaning device to separate the support layer from the filter layer whereby friction between the filter layer and the powder cleaning device is greatly reduced to extend the useful life of the movable belt.
Oversized parts generally require large openings to allow the parts to pass through the booths. To insure that the toxic powder does not drift out of the booth openings and into adjacent areas, the system requires a large amount of air to flow through the booth. It is not unusual for the booth to draw in over 12,000 cubic feet per minute (CFM) of air. Most filter belt media will only pass 50 CFM per square foot. Belts designed for larger air flow cannot prevent the fine powder from passing through the media. Thus a 12,000 CFM booth would require 240 square feet of filter area. A six foot wide flat filter belt would require a 40 foot long booth. A larger booth costs more to install, takes longer to clean and requires a lot of factory floor area. It would be desirable to have a filter belt assembly that improves the recovery efficiency of excess powder from the inner cavity of a spray booth in a shorter booth length.